From spiropentane to butterfly and tetrahedral structures in tetranuclear iron carbonyl carbide chemistry

摘要

Oxidative degradation of the octahedral dianion [Fe6C(CO)_(16)]~(2-) with an interstitial carbon atom leads eventually to the neutral Fe4C(CO)_(13) cluster with a butterfly-shaped central Fe4C unit. The complete series of related Fe4C(CO)_n (n = 16, 15, 14, 13, 12, 11) derivatives have now been investigated using density functional theory. For the lowest energy Fe4C(CO)_n (n = 16, 15, 14, 13) structures the geometries obey the n + f = 18 rule where f is the number of Fe-Fe bonds. This leads to a spiropentane geometry with two Fe-Fe bonds for Fe4C(CO)_(16), a central bent Fe-Fe-Fe-Fe chain for Fe4C(CO)_(15), a distorted trigonal pyramidal structure with four Fe-Fe bonds for Fe4C(CO)_(14), and the experimentally observed butterfly structure with five Fe-Fe bonds for Fe4C(CO)_(13). A symmetrical higher energy centered tetrahedral structure for Fe4C(CO)_(12) with six Fe-Fe bonds also follows the n + f = 18 rule. However, the lowest energy Fe4C(CO)_n [n = 12, 11) structures are derived from the lowest energy Fe4C(CO)_(13) structure by removal of CO groups with retention of the central Fe4C butterfly unit.